For a long time specialist literature has described many heat-curable compounds filled with pulverulent inorganic materials of preferably pigment-like size and the processes for obtaining them, which concern both foamed and unfoamed filled heat-curable compounds, which in the first case are in the form of plastic masses containing fine gaseous inclusions.
The introduction of inorganic fillers (French patent 2,531,971) into the various heat-curable compounds of the polyurethane type is directed at various objectives in order to respond to demands of specialist or user industries, such as, for example, the motor vehicle, building, electronics, electrical domestic appliances or other, industries, for availability of nonmetallic products with specific characteristics such as lightness, rigidity, reduced shrinkage, decrease in the expansion coefficient, improvement in thermal shock resistance, improvement in sound insulation, sufficient flexural, tensile, compressive mechanical strength or other major physical characteristics.
Various fillers intended for heat-curable compounds of the polyurethane type are referred to in "European Plastics News" (1979, August, page 21) and "Modern Plastic International" (1982, April, page 42), such as calcium carbonate, talc, mica, aluminium trihydrate, silica, but also glass fibres, textile fibres or others. Thus, in the RIM (Reaction Injection Moulding) and RRIM (Reinforced Reaction Injection Moulding) processes fillers as diverse as glass fibres, ballotini, mica, wollastonite, talc and treated inorganic fibres are often used in order to increase the rigidity of the articles produced and to reduce their cost ("Plastic Technologic" 1978, November, page 13 and "Elastomerics", 1979, February, page 25).
Various processes are proposed for performing the introduction of fillers into heat-curable compounds, the essential preoccupation of which is to produce filled heat-curable compounds which have at least some of the specific characteristics referred to above.
In a first type of process the filler is introduced into one of the constituents of the polyurethane compound, generally the polyol. To stabilize the suspension thus prepared, that is to say to avoid sedimentation effects, the polyol is frequently subjected to grafting (DE OS 2,654,746, 2,714,291 and 2,739,620), with methacrylic acid or with another vinyl compound such as styrene, or else with isocyanate (DE OS 2,834,623). However, experimental results show that the suspension thus prepared does not escape either a considerable increase in its viscosity, which makes it difficult to handle, or a poor distribution of the filler within the heat-curable compound produced.
According to a second type of process, and to try and get rid of the disadvantages revealed in the first type, the filler is surface-treated before its introduction into one of the constituents of the desired heat-curable compound (the polyol), by means of a coating agent which is compatible with the constituents, a coating agent which is, for example, a C.sub.8 to C.sub.14 alcohol (FR 2,531,971). However, this type of process produces disadvantages which are substantially identical with those referred to earlier, since the user still finds an increase in the viscosity of the suspension of the filler in the polyol, attenuated to be sure, but still too high, which causes nonuniform dispersion of this filler in the heat-curable compound which is subsequently formed.
In another type of process, which is directed towards the production of unfoamed filled heat-curable compounds (intended to be compression-moulded), the process consists in preparing (for example according to FR 1,512,029) a mixture consisting of the filler and of the necessary additives such as lubricants, condensation catalysts, optionally colouring substances and plasticizers and in then introducing into this mixture, which is being stirred vigorously, the heat-curable resin or its initial constituents, in well-defined quantities and proportions.
However, the heat-curable compounds thus produced exhibit the same disadvantages as those already referred to, which result from a nonuniform dispersion of the filler within the said compounds.
According to another type of process which is directed more towards the production of filled foamed heat-curable compounds, the filler and the plasticizer are introduced separately into the mixture of the constituents of the desired heat-curable compound. A process of this type (described for example in JP 56-155,232 or ES 371,150) results in foamed heat-curable compounds of the polyurethane type in which the cells are formed nonuniformly and are consequently embrittled by the poor filler dispersion.
Thus, it appears that a real and serious problem exists as soon as fillers, in particular inorganic ones, are introduced into polyurethane heat-curable compounds, since these filled compounds exhibit unacceptable physical anomalies.
In the case of unfoamed filled polyurethane compounds the physical anomalies revealed by differences from these same compounds unfilled are, for example, not only the increase in the density and in the rigidity, but also the weakening in mechanical characteristics such as reduced tensile, flexural and compressive strength.
In the case of filled and foamed polyurethane compounds, physical anomalies revealed in the first group also manifest themselves, with the additional appearance of phenomena such as the increase in the density of the filled compound, in its heterogeneous hardness to the touch, in nonuniformity of flexibility and in mechanical characteristics which are rapidly weakened by the application of repeated deformation cycles (hysteresis). All these phenomena are linked, as the Applicant Company has been able to ascertain, with a poor dispersion of the filler within the desired heat-curable compound.
Lastly, added to all these ills are phenomena of increase in the viscosity of the medium receiving the filler or of sedimentation of the said filler in industrial plants, phenomena which prejudice their good operation and the quality of the heat-curable compounds to be marketed.
Consequently, the problem of the utilization of the fillers in the heat-curable compounds of the polyurethane type remaining in its entirety, it is understandable that their use in these compounds is very limited.
Thus, where the polyurethanes are concerned, the objectives which the invention is aimed at are to prevent the increase in the viscosity of the constituent or of the mixture of the constituent of the desired heat-curable compound receiving the inorganic filler; to prevent the filler sedimentation phenomena in industrial plants suitable for the production of the desired heat-curable compound; to permit the introduction of the filler into one of the constituents or simultaneously with all the constituents or else in the mixture of the constituents of the desired heat-curable compound; finally, to produce an excellent distribution of the inorganic filler within the desired heat-curable compound, in order to provide it with an appreciable improvement in its physical and mechanical properties and processability, while increasing the quantity of filler which is introduced.